1. Field of the Invention
The present invention relates to an image-projecting apparatus such as a liquid crystal projector, etc. In particular, the invention relates to a technology for detecting a focusing state by receiving reflected light when projecting light onto a projection surface, which is a focusing detection technology in the case where the light to be detected is reflection light which derives from image projecting light of the image-projecting apparatus.
2. Description of Related Art
Conventionally, as an AF (Auto-Focusing) method of a projector, some methods have been known, one of which is an active method which performs auto-focusing based on a principle of triangulation-optical type focusing detection by using infrared light, and another of which is a passive method which reads a contrast of brightness on a screen by means of a pair of light-receiving sensors and driving a lens by obtaining correlation values of respective pixels output and obtaining a distance.
Also, the passive method of the AF method has two types classified in a large category, one of which is a so-called two-image correlation (or, displacement detection) AF method (Refer to Japanese Patent No. 3120526) which an parallax image such as an AF chart being an object for focusing detection is formed on a plurality of photo electric sensors disposed in a predetermined base length and performs obtaining and comparing of image information, and another method (hill climbing) which obtains the optimal state of focusing by imaging an object for focusing detection on a multi-pixel optical sensor having a one-dimensional or two-dimensional pixel arrangement and detects the image sharpness (contrast) from the image information, and driving the focusing position of a lens for forming the object image in the sensor.
In addition to the above, by using characteristics of a projector itself projecting an image, a technology which detects a specifically projected image by a camera and detects the focusing by comparing an original image information projected with an image-taking information (Refer to Japanese Patent Application Laid-Open No. 2000-28901).
Where projection light of such a projector itself is used for focusing adjustment, processing is performs under the same conditions as advantageous conditions of brightness regardless although disadvantageous brightness for the passive AF, the passive AF method can be performing unnecessarily erroneous detecting operation of a focusing.
For this reason, a number of technologies which change such as a determination algorithm, a focusing detecting operation and a focusing detecting method in accordance with a target object and environmental brightness, when focusing detection is performed (Refer to Japanese Patent No. 3272429 and Japanese Patent Application Laid-Open No.2001-317935).
In Japanese Patent No. 3272429, a focusing detecting apparatus which comprises a light emission unit which emits a light projected to a target object, a light emission output controller which controls light emitting output of the light emission, a first detector which receives reflected light from the target object and measuring the distance from the light-receiving position to the target object, and a determining unit which determines a dispersion of the distance measured a plurality of times by the first detector and the light emission output controller increases the light emitting output from the light emission unit when the dispersion determined by the determining unit exceeds a predetermined value.
Also, Japanese Patent Application Laid-Open No.2001-317935 relates to a distance-detector apparatus which detects the distance to a target object by receiving reflected light from the target object with light projected, the distance-detector apparatus comprises a detector which detects a light-receiving intensity level of the reflected light and a controller which determines the number of times of projecting and receiving light in accordance with the detected light-receiving intensity level and controls for performing repeated projected and receiving by the number of times. In the distance-detector apparatus, the lower the light-receiving intensity level becomes, the greater the number of times of projecting and receiving light increased, and measurement result information are calculated by executing an average processing calculation on the basis of information obtained by projecting and receiving by a plurality of times.
However, the respective technologies are based on an infrared LED as a projection light source and an apparatus driven a battery. The respective technologies do not disclose any technology to improve the AF accuracy, which is suitable for a distance detecting or focusing detecting method used for an image-projecting apparatus such as a liquid crystal projector, etc.
Specifically, as has been proposed in Japanese Patent No. 3272429, in the case where the light emitting output of the light emission unit is increased in auto focusing (AF), in the image-projecting apparatus such as a liquid crystal display, etc., it is necessary to employ a lamp suitable for output control as a lamp for illuminating image-forming elements such as a liquid crystal panel, etc., thus problems occur such a costs are increased and it is not realistic.
Further, as described above, in the image-projecting apparatus, auto focusing of a projection optical system is performed on the basis of signals output from a sensor by forming reflected light from a projection screen onto the sensor. However, if the original brightness of a screen on which no image is projected is high (that is, environmental brightness: which becomes brightness at an area other than the projection screen during projecting an image) even if the brightness of a projection surface is high, a contrast suitable for distance detection (or focusing detection) on the screen cannot be obtained.
Therefore, in a case where a distance-detecting apparatus proposed by Japanese Patent Application Laid-Open No. 2001-317935 is applied to an image-projecting apparatus and only distance detection (or focusing detection) is performed by the number of times in accordance with the light-receiving intensity level of light (sum of projection light and environmental light) from a projection surface, there are cases where a contrast suitable for distance detection (or focusing detection) cannot be obtained on the sensor, and it is impossible to securely improve the AF accuracy.